Yttrium-doped barium fluoride crystal and preparation method and use thereof

ABSTRACT

Disclosed are a yttrium-doped barium fluoride crystal and a preparation method and the use thereof, wherein the yttrium-doped barium fluoride crystal has a chemical composition of Ba (1−x) Y x F 2+x , in which 0.01≤x≤0.50. The yttrium-doped BaF 2  crystal of the present invention has improved scintillation performance. The yttrium doping may greatly suppress the slow luminescence component of the BaF 2  crystal and has an excellent fast/slow scintillation component ratio. The doped crystal is coupled to an optical detector to obtain a scintillation probe which is applicable to the fields of high time resolved measurement radiation such as high-energy physics, nuclear physics, ultrafast imaging and nuclear medicine imaging.

TECHNICAL FIELD

The present application relates to a barium fluoride (BaF₂) crystal, inparticular to a yttrium-doped barium fluoride crystal, and a preparationmethod and use thereof.

BACKGROUND

Inorganic scintillation crystals are a kind of photo-functional crystalmaterial capable of converting the absorbed energy of incident energeticrays or particles into pulses of light. The decay time is a dynamicparameter showing the intensity of the pulsed light emitted by ascintillation crystal after absorbing energy as a function of time, andcharacterizes the response rate characteristics of a scintillationmaterial to energetic rays or particles. It has always been importantresearch interests and hotspot to develop fast scintillators. Bariumfluoride (BaF₂) crystal is currently known as the fastest inorganicscintillator, having a fast scintillation component peaked at 195 nm and220 nm with a decay time of less than 1 ns. The light output of BaF₂crystal is about twice that of LSO/LYSO:Ce crystal within the initial0.1 ns of luminescence process. BaF₂ crystal has a good radiationresistance and a relatively low price. From the end of the 1980s to theearly 1990s, this unique scintillation crystal has been widely used asone of important candidates for electromagnetic calorimeters in manyparticle physics experiments, and attracted much attention. However,besides the fast scintillation component, this crystal has also a slowscintillation component peaked at 300 nm with a decay time of about 600ns and with a light output of four to five times that of the fastscintillation component. During the measurement at a high count rate(>10⁶ Hz), the slow component would cause serious signal pile-up, whichgreatly limits its application in fields such as high rate counting andtime-resolved radiation measurement.

Suppression of the slow scintillation component of BaF₂ crystal is abasic prerequisite for the wide application of the crystal. There aremainly three approaches to suppress the impact of slow scintillationcomponent: the first is the use of photodetectors sensitive only to fastscintillation component, such as a photomultiplier tube using TMAE,Cs—Te, K—Cs—Te or Rb—Te as a photocathode material, an avalanchephotodiode, or a silicon photomultiplier tube, etc.; the second is toreduce the light output of the slow scintillation component byincreasing crystal temperature or by selective doping; and the third isto regulate the transmission of light between the crystal and thephotodetector, such as the separation of the fast scintillationcomponent by organic shifting materials, and the selective filtering ofthe slow scintillation component by photonic crystal structure andreflective VUV bandpass filters.

Selective doping, i.e., incorporation a certain amount of other ionssuch as La³⁺ ions into BaF₂ crystal to make the luminescence intensityof the slow scintillation component weaker, is a more practicalapproach, and has attracted sustained attention over the past threedecades. Nowadays, with the latest fast component-sensitive detectorsand band-pass filter apparatus, selective doping is expected to driveBaF₂ crystal to a wide range of applications. In 1989, P. Schotanus etal. found that introducing a certain concentration of La³⁺ ions intoBaF₂ crystals can significantly attenuate the luminescence intensity ofthe slow scintillation component, while the fast scintillation componentof BaF₂ remains unaffected. C. L. Woody et al. found that La-dopingpreserves the irradiation hardness of BaF₂ crystal. From then on, theLa-doping has received the most extensive attention, and the research onthe doping amount optimization, suppression characteristics, andsuppression mechanism of La-doping has made great progress. Although itis controversial that the mechanism of slow component suppression iswhether the reduction of the dissociation energy of STE due tointerstitial F⁻ ion, the reduction of the number of STE due toLa-doping, or the formation of H center that does not contribute to STEluminescence due to the combination of the V_(k) center and theinterstitial F⁻ ion, the conclusions that La-doping can suppress theslow component are consistent.

Unfortunately, although La-doping can suppress the slow scintillationcomponent of BaF₂ crystal, the preparation of La-doped BaF₂ crystal hasgreat technical challenges, and La-doping will inevitably introduce thebackground radioactivity of ¹³⁸La isotope, which limits the wide use ofLa³⁺ as a slow component suppression ion. It is urgent to search otherslow component suppression doping ions for easier growth of large-sizedoped crystals with high optical quality, to promote the substantialapplication of BaF₂ crystal in the high time-resolved fields.

SUMMARY Technical Problem

In view of the above problems, an object of the present application isto provide a yttrium-doped barium fluoride crystal with suppressed slowscintillation component, and a preparation method and applicationthereof, to remarkably improve the time resolved characteristics ofbarium fluoride crystals.

TECHNICAL SOLUTION

In one aspect, the present application provides a yttrium-doped bariumfluoride crystal, the yttrium-doped barium fluoride crystal having achemical composition of Ba_((1−x))Y_(x)F_(2+x), wherein 0.01≤x≤0.50.

According to the physical property of Y³⁺ ion that the ionic radius andelectronegativity thereof are similar to those of La³⁻ ion, a certainconcentration (1 to 50 at %) of Y³⁺ ions is introduced into a BaF₂crystal matrix, thereby interstitial fluoride ions F_(i) ⁻ areintroduced into the crystal lattice after the Y³⁺ ions entering the BaF₂crystal matrix, to destroy the self-trapped exciton luminescenceprocess, so that the luminescence intensity of the slow scintillationcomponent is weakened. Because the melting point of LaF₃ (1493° C.) ismuch higher than that of BaF₂ (1368° C.), a La-doped BaF₂ crystal grownis prone to having macroscopic defects such as bubbles and inclusions.The growth of high optical quality La-doped crystals has always been agreat challenge. The melting point of YF₃ (1387° C.) is very close tothat of BaF₂. As compared with La-doping, Y-doping is much easier toachieve precise control of doping stoichiometry, and the doping does notincrease the difficulty of crystal growth. Y-doping does not introducethe radioactive background of the ¹³⁸La isotope, thus the yttrium-dopedbarium fluoride crystal can be used in the field of low-backgroundradiation detection. The density of YF₃ (4.01 g/cm³) is lower than thatof LaF₃ (5.9 g/cm³), thus the mass of the YF₃ dopant is 47% less thanthe that of the LaF₃ dopant at the same doping stoichiometric ratio,making Y³⁺ doping have a significant cost advantage.

Preferably, the yttrium-doped barium fluoride crystal may be used inmonocrystalline or a polycrystalline state.

In another aspect, the present application provides a method forpreparing the yttrium-doped barium fluoride crystal, comprising thesteps of:

weighing and mixing raw materials of YF₃ and BaF₂ according to the molarratio BaF₂: YF₃=(1−x): x to obtain a mixed powder, wherein 0.01≤x≤0.50;

putting the mixed raw materials into a crucible in a vacuum furnace forthorough melting and mixing, and then cooling the mixture to obtainBa_((1−x))Y_(x)F_(2+x) polycrystalline material, or subjecting the mixedpowder to isostatic pressing, and putting the resulting substance intocrucibles and sintering it at 900 to 1200° C. in vacuum to obtainsintered Ba_((1−x))Y_(x)F_(2+x) polycrystalline material; and

mixing the resulting polycrystalline material with a appropriate amountof PbF₂ powder, and growing crystals by a melt method.

Preferably, the growth method may include vertical Bridgman method orCzochralski method.

Preferably, the growth processes of the vertical Bridgman method mayinclude:

maintaining a vacuum degree of less than 10⁻³ Pa, melting theBa_((1−x))Y_(x)F_(2+x) polycrystalline material and PbF₂ powder at 1200to 1400° C., subjecting the resulting melt to crystal growth wherein thedescending speed of the crucible is 0.5 to 4 mm/hour, and cooling thegrown crystal to room temperature at a temperature decreasing rate of 10to 50° C./hour.

Preferably, the crucible may be a high purity graphite crucible or aglassy carbon crucible.

Preferably, the isostatic pressing may be performed at a pressure of 5to 20 MPa for 0.1 to 1 hour, and the temperature for the thoroughmelting is 1200 to 1400° C.

Preferably, the deoxidizer PbF₂ may be added in an amount of 0.1 to 5 wt%, preferably 0.5 to 2 wt %, by weight of the Ba_((1−x))Y_(x)F_(2+x)polycrystalline material.

In the third aspect, the present application provides a scintillationcrystal probe, comprising the above-described yttrium-doped bariumfluoride crystal, and a photomultiplier tube, an avalanche photodiode ora silicon photomultiplier tube coupled to the yttrium-doped bariumfluoride crystal. The yttrium-doped barium fluoride crystal may be usedin monocrystalline state, or may be in polycrystalline state, which isuniformly dispersed in a transparent medium, or in crystal array stateformed by a plurality of crystal elements.

In the fourth aspect, the present application provides use of theabove-described yttrium-doped barium fluoride crystal in the field ofhigh time-resolved radiation detection.

The yttrium-doped barium fluoride crystal with a high suppression ratioof the slow component prepared herein can be used in the fields of hightime-resolved radiation detection. These fields include, but are notlimited to, high energy physics, nuclear physics, nuclear medicineimaging, X-ray imaging, etc. The yttrium-doped barium fluoride crystalis used in the form of monocrystalline or polycrystalline in thesefields.

As compared with BaF₂ crystal with La-doping, the —BaF₂ crystal withY-doping of the present application also has an excellent fast/slowscintillation ratio, is much easier to grow, does not introduce theradioactive background of ¹³⁸La isotope, and needs less amount ofdopants at the same doping stoichiometric ratio, thus having asignificant comparative advantage. The yttrium-doped barium fluoridecrystals of the present application are suitable for use in the fieldsof high time-resolved radiation detection.

The X-ray excitation emission spectra of undoped/pure BaF₂ and 1 at %Y-doped BaF₂ crystal at room temperature are shown in FIG. 1. As can beseen from FIG. 1, the luminescence intensity of the slow scintillationcomponent peaked at 300 nm in the X-ray excited emission (XEL) spectrumof —BaF₂ crystal with Y-doping changes significantly as compared to theundoped BaF₂ crystal. FIG. 2 shows the comparison of light output anddecay kinetic characteristics of undoped/pure BaF₂ (top) and 1 at %Y-doped BaF₂ crystal (bottom) with dimensions of 30*30*20 mm³ atdifferent integrate time. It can be seen that the fast scintillationcomponent of 1 at % Y-doped BaF₂ crystal is equivalent to that of theundoped BaF₂ crystal, while the slow scintillation component is reducedfrom 906 ph./MeV to 146 ph./MeV, and the fast/slow component ratio isincreased from 0.2 to 1.3, and the slow component suppression ratio isup to 6.44. Under the same process conditions, —it easier to growlarge-sized BaF₂ crystals with Y-doping. FIG. 3 shows a Y-doped BaF₂crystal with a length of 200 mm, which can meet the requirement forlarge-size BaF₂ crystals in high energy physical scientific facilitiesat the intensity frontiers, such as Mu2e, Project X, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the X-ray excited emission (XEL) spectra of undoped BaF₂and 1 at % Y-doped BaF₂ crystal at room temperature, wherein the solidline represents the spectrum of undoped BaF₂, and the dash linerepresents the spectrum of Y-doped BaF₂ crystal;

FIG. 2 shows the light output and decay kinetic characteristics ofundoped BaF₂ and 1 at % Y-doped BaF₂ crystal at different integratetime;

FIG. 3 shows an 1 at % Y-doped BaF₂ crystal with a length of 200 mm;

FIG. 4 shows a scintillation crystal probe composed of an undoped BaF₂crystal and a photomultiplier tube R2059;

FIG. 5 shows a scintillation crystal probe composed of a Y-doped BaF₂crystal and a photomultiplier tube R2059;

FIG. 6 shows a scintillator probe composed of a Y-doped BaF₂microcrystalline/organic composite scintillator and APD; and

FIG. 7 shows a scintillation crystal probe composed of a Y⁻doped BaF₂crystal and a SiPM.

DETAILED DESCRIPTION

The present invention will be further described with the followingembodiments below. It should be understood that the followingembodiments are only used for explaining this invention, but not tolimit this invention.

The present application relates to the improvement of the scintillationperformance, especially time response characteristics of BaF₂ crystal.Yttrium-doping can greatly suppress the slow scintillation component ofBaF₂ crystal. The yttrium-doped barium fluoride crystal has a chemicalcomposition of Ba_((1−x))Y_(x)F_(2+x), wherein x represents the dopingconcentration of the yttrium, and 0.01≤x≤0.50. If the dopingconcentration of the yttrium is too high, the cost of the crystal willbe greatly increased, and the density of the doped crystal will belowered, which is unfavourable for the radiation detecting efficiency.Preferably, 0.01≤x≤0.10. The yttrium-doped barium fluoride crystal maybe in monocrystalline or polycrystalline state. The yttrium-dopedcrystal can be used in the fields of high time-resolved radiation suchas high energy physics, nuclear physics, ultrafast imaging, nuclearmedicine imaging, etc.

In the present application, the raw materials are thoroughly mixedaccording to the molar ratio of BaF₂:YF₃=1−x (x=0.01-0.50), and anappropriate amount of PbF₂ is added as a deoxidizing agent. Theresulting mixture is subjected to crystal growth by using verticalBridgman furnace in vacuum. The preparation method of the yttrium-dopedbarium fluoride crystal provided by the present application will beexemplified below.

Preparation of Ba_((1−x))Y_(x)F_(2+x) polycrystalline material. Rawmaterials of YF₃ and BaF₂ are weighed and mixed according to the molarratio BaF₂: YF₃=(1−x): x to obtain a mixed material. Specifically, BaF₂powder having a purity of 99.99% or more and YF₃ powder having a purityof 99.9% or more are used as raw materials, and these raw materials arefully dried in a vacuum oven at 150 to 200° C. The dried raw materialsare weighed according to the molar ratio of BaF₂:YF₃=(1−x): x (wherein xis 0.01 to 0.50), an appropriate amount of PbF₂ powder is weighed as adeoxidizer, and BaF₂, YF₃ and PbF₂ are thoroughly mixed to obtain amixed powder.

The mixed powder is fed into a crucible, thoroughly melted and mixed ina vacuum furnace at 1200 to 1400° C., and cooled, to obtain aBa_((1−x))Y_(x)F_(2+x) polycrystalline material. As an example, themixture is fed into a high-purity graphite crucible or a glassy carboncrucible, and then the mixture is thoroughly and mixed in a vacuumfurnace to obtain a BaF₂—YF₃ solid solution melt, and the solid solutionmelt is cooled to obtain Ba_((1−x))Y_(x)F_(2+x) polycrystallinematerial.

Alternatively, the mixed powder is subjected to isostatic pressing, fedinto a crucible, and then sintered at 900 to 1200° C. in vacuum toobtain Ba_((1−x))Y_(x)F_(2+x) polycrystalline material. The isostaticpressing may be performed at a pressure of 5 to 20 MPa for 0.2 to 2hours. The crucible may be a high purity graphite one or a glassy carbonone. As an example, the mixed raw materials are put into a plastic bagand isostatically pressed in an isostatic press, and then transferredinto a high-purity graphite or a glassy carbon crucible, placed in avacuum furnace for sintering at a temperature of 900 to 1200° C., andcooled, to obtain Ba_((1−x))Y_(x)F_(2+x) polycrystalline material.

The Ba_((1−x))Y_(x)F_(2+x) polycrystalline material is mixed with anappropriate amount of PbF₂ powder, and subjected to crystal growth by amelt method. The melt method includes, but is not limited to, verticalBridgman method and Czochralski method. The deoxidizer PbF₂ may be addedin an amount of 0.1 to 5 wt %, preferably 0.5 to 2 wt %, of theBa_((1−x))Y_(x)F_(2+x) polycrystalline material.

The processes of the vertical Bridgman method include: maintaining avacuum degree of less than 10⁻³ Pa, melting the Ba_((1−x))Y_(x)F_(2+x)polycrystalline material and PbF₂ powder at 1200 to 1400° C., subjectingthe resulting melt to start the crystal growth, wherein the descendingspeed of the crucible is 0.5 to 4 mm/hour, and cooling the grown crystalto room temperature at a temperature decreasing rate of 10 to 50°C./hour. Specifically, a high-purity graphite crucible or a glassycarbon crucible having a capillary structure at the bottom is machinedaccording to the size and number of crystals to be grown, and theBa_((1−x))Y_(x)F_(2+x) polycrystalline material and an appropriateamount of PbF₂ powder are fed into the graphite crucibles or the glassycarbon crucibles, and placed into a vertical vacuum Bridgman furnace. Avacuum pumping device is turned on so that the vacuum inside the furnaceis less than 10⁻³ Pa, and then the temperature is gradually increased tothoroughly melt the raw material, and a descending device is turned onfor crystal growth, wherein the descending speed is 0.5 to 4 mm/h. Afterthe growth is completed, the crystal is cooled to room temperature at atemperature decreasing rate of 10 to 50° C./hour, and as-grown crystalingot is taken out for machining.

The yttrium-doped crystal in the present application can be coupled to aphotodetector such as a photomultiplier tube, an avalanche photodiode,and a silicon photomultiplier tube for use in the field of hightime-resolved radiation detection. The present application relates tothe improvement of the scintillation performance, especially timeresponse characteristics of BaF₂ crystal. Yttrium doping can greatlysuppress the slow scintillation component of BaF₂ crystal. That is, theyttrium-doped barium fluoride crystal of the present application has anexcellent fast/slow scintillation ratio, and the yttrium-doped crystalcan be coupled to a photodetector to form a scintillation probe, whichis applicable to the field of high time-resolved radiation, includingbut not limited to, high energy physics, nuclear physics, ultrafastimaging, nuclear medicine imaging, etc.

Hereinafter, the present invention will be better described with thefollowing representative examples. It should be understood that thefollowing examples are only used to explain this invention and do notlimit the scope of this invention. Any non-essential improvements andmodifications made by a person skilled in the art based on thisinvention are all protected under the scope of this invention. Thespecific parameters below are only exemplary, and a person skilled inthe art can choose proper values within an appropriate range accordingto the description of this article, and are not restricted to thespecific values cited below. It should be noted that the embodimentsdescribed below are only for explaining the application, and are a partof but not all of the embodiments of the application. Based on theembodiments of the present application, all other embodiments obtainedby those skilled in the art without creative efforts are within theprotection scope of the application.

Example 1

Preparation of 1 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.9% wereused as the starting materials. These starting materials were weighed ina molar ratio of BaF₂:YF₃=0.99:0.01, and heated in a vacuum oven at 200°C. for 20 hours. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a high-purity graphitecrucible, and then thoroughly melted in a vacuum furnace at 1300° C. toobtain a BaF₂—YF₃ solid solution melt. The melt was cooled to roomtemperature to obtain Ba_(0.99)Y_(0.01)F_(2.01) polycrystallinematerial.

3) A high-purity graphite crucible or a glassy carbon crucible having acapillary structure at the bottom was machined according to the size andnumber of crystals to be grown, and the Ba_(0.99)Y_(0.01)F_(2.01)polycrystalline material and an appropriate amount of PbF₂ powder werefed into the graphite crucible, and placed into a vacuum crucibledescending furnace, wherein the deoxidizer PbF₂ was added in an amountof 0.5 wt % of the Ba_((1−x))Y_(x)F_(2+x) polycrystalline material.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to 1300° C. to thoroughly melt the raw material, anda descending device was turned on for crystal growth, wherein thedescending speed was 2 mm/h. After the growth was completed, the crystalwas cooled to room temperature at a temperature decreasing rate of 50°C./hour, and the crystal ingot was taken out for machining.

Example 2

Preparation of 10 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.99% wereused as the starting materials. These starting materials were fullydried in a vacuum oven, and weighed in a molar ratio ofBaF₂:YF₃=0.90:0.10. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a glassy carbon crucible, andthen thoroughly melted in a vacuum furnace at 1350° C. to obtain aBaF₂—YF₃ solid solution melt. The melt was cooled to room temperature toobtain Ba_(0.9)Y_(0.1)F_(2.1) polycrystalline material.

3) A high-purity graphite crucible having a capillary structure at thebottom was machined according to the size and number of crystals to begrown, and the Ba_(0.9)Y_(0.1)F_(2.1) polycrystalline material and anappropriate amount of PbF₂ powder were fed into the graphite crucible,and placed into a vacuum crucible descending furnace, wherein thedeoxidizer PbF₂ was added in an amount of 1 wt % of theBa_((1−x))Y_(x)F_(2+x) polycrystalline material.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to 1350° C. to thoroughly melt the raw material, anda descending device was turned on for crystal growth, wherein thedescending speed was 1 mm/h. After the growth was completed, the crystalwas cooled to room temperature at a temperature decreasing rate of 25°C./hour, and the crystal ingot was taken out for machining.

Example 3

Preparation of 20 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.99% wereused as the starting materials. These starting materials were fullydried in a vacuum oven, and weighed in a molar ratio ofBaF₂:YF₃=0.80:0.20. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a plastic bag andisostatically pressed in an isostatic press, and then placed in a vacuumfurnace for vacuum sintering at a temperature of 900 to 1200° C., andcooled, to obtain Ba_(0.8)Y_(0.2)F_(2.2) polycrystalline material,wherein the isostatic pressing treatment was performed at a pressure of20 MPa for 0.5 hour.

3) Alternatively, the mixture was fed into a plastic bag andisostatically pressed in an isostatic press, and then transferred into ahigh-purity graphite or a glassy carbon crucible, placed in a vacuumfurnace for sintering at a temperature of 1000° C., and cooled, toobtain Ba_(0.8)Y_(0.2)F_(2.2) polycrystalline material.

4) The Ba_(0.8)Y_(0.2)F_(2.2) polycrystalline material and anappropriate amount of PbF₂ powder were fed into a glassy carbon cruciblehaving a capillary structure at the bottom and having an inner diameterof 80 mm. The glassy carbon crucible filled with the raw materials wasplaced in a vacuum crucible descending furnace. The deoxidizer PbF₂ wasadded in an amount of 1.5 wt % of the Ba_((1−x))Y_(x)F_(2+x)polycrystalline material.

5) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to 1250° C. to thoroughly melt the raw material, anda descending device was turned on for crystal growth, wherein thedescending speed was 1 mm/h. After the growth was completed, the crystalwas cooled to room temperature at a temperature decreasing rate of 10°C./hour, and the crystal ingot with a diameter of 80 mm was taken outfor machining.

Comparative Example 1

Preparation of Pure (Undoped) BaF₂ Crystal

1) BaF₂ having a purity of 99.99% was used as the starting material, andheated in a vacuum oven at 200° C. for 20 hours. An appropriate amountof PbF₂ was weighed as a deoxidizer. BaF₂ and PbF₂ were thoroughly mixedto obtain a BaF₂—PbF₂ mixture.

2) The BaF₂—PbF₂ mixture was fed into a high-purity graphite crucible,and then thoroughly melted in a vacuum furnace at 1300° C. to obtain aBaF₂—YF₃ solid solution melt. The melt was cooled to room temperature toobtain BaF₂ polycrystalline material.

3) A high-purity graphite crucible or a glassy carbon crucible having acapillary structure at the bottom was machined according to the size andnumber of crystals to be grown, and the BaF₂ polycrystalline materialand an appropriate amount of PbF₂ powder were fed into the graphitecrucible, and placed into a vacuum crucible descending furnace, whereinthe deoxidizer PbF₂ was added in an amount of 0.5 wt % of the BaF₂polycrystalline material.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to 1300° C. to thoroughly melt the raw material, anda descending device was turned on for crystal growth, wherein thedescending speed was 2 mm/h. After the growth was completed, the crystalwas cooled to room temperature at a temperature decreasing rate of 20°C./hour, and the crystal ingot was taken out for machining.

Use of Pure BaF₂ Crystal in Radiation Detection

The crystal ingot obtained in Comparative Example 1 was machined into aBaF₂ crystal having a size of 30*30*20 mm³. A Hamamatsu R2059photomultiplier tube (PMT) was coupled to one 30*30 mm² end surface ofthe crystal with a coupling silicone grease (Dow Corning XIAMETER®PMX-200), and the other surfaces were wrapped with Tyvek, to form ascintillation crystal probe as shown in FIG. 4.

Example 4

Use of Y-Doped BaF₂ Crystal in Radiation Detection

The crystal ingot obtained in Example 1 was machined into a 1 at %Y-doped BaF₂ crystal having a size of 30*30*20 mm³. One 30*30 mm² endsurface of the crystal was coupled to a Hamamatsu R2059 photomultipliertube (PMT) with a coupling silicone grease (Dow Corning XIAMETER®PMX-200), and the other surfaces of the crystal were wrapped with Tyvek,to form a scintillation crystal probe as shown in FIG. 5. The probe hasexcellent slow component suppression and time-resolved properties, andcan be used in radiation detection such as high energy physics, nuclearphysics, nuclear medicine imaging, X-ray imaging, etc.

Example 5

Use of Y-Doped BaF₂ Crystal in Radiation Detection

The crystal obtained in Example 2 was ground into a monocrystallinepowder and uniformly dispersed in a high ultraviolet ray-transmissiveepoxy resin to prepare a composite scintillator having a size of Φ5*5mm³. One Φ5 mm of the crystal was coupled to a UV-sensitive avalanchephotodiode (APD) to with a coupling silicone grease (Dow CorningXIAMETER® PMX-200), and the other surfaces of the crystal were wrappedwith Teflon tape, to form a scintillation crystal probe as shown in FIG.6. The probe has excellent slow component suppression and time-resolvedproperties, and can be used in radiation detection such as high energyphysics, nuclear physics, nuclear medicine imaging, X-ray imaging, etc.

Example 6

Use of Y-Doped BaF₂ Crystal in Radiation Detection

The crystal ingot obtained in Example 2 was machined into a Y⁻doped BaF₂crystal having a size of 10*10*10 mm³. One 10*10 mm² surface of thecrystal was coupled to a silicon photomultiplier (SiPM) with a couplingsilicone grease (Dow Corning XIAMETER® PMX-200), and the other surfacesof the crystal were wrapped with Tyvek, to form a scintillation crystalprobe as shown in FIG. 7. The probe has excellent slow componentsuppression and time-resolved properties, and can be used in radiationdetection such as high energy physics, nuclear physics, nuclear medicineimaging, X-ray imaging, etc.

Example 7

Preparation of 30 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.99% wereused as the starting materials. These starting materials were fullydried in a vacuum oven, and weighed in a molar ratio ofBaF₂:YF₃=0.70:0.30. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a glassy carbon crucible, andthen thoroughly melted in a vacuum furnace at 1350° C. to obtain aBaF₂—YF₃ solid solution melt. The melt was cooled to room temperature toobtain Ba_(0.7)Y_(0.3)F_(2.3) polycrystalline material.

3) A high-purity graphite crucible having a capillary structure at thebottom was machined according to the size and number of crystals to begrown, and the Ba_(0.7)Y_(0.3)F_(2.3) polycrystalline material and anappropriate amount of PbF₂ powder were fed into the graphite crucible,and placed into a vacuum crucible descending furnace.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to thoroughly melt the raw material, and adescending device was turned on for crystal growth, wherein thedescending speed was 1 mm/h. After the growth was completed, the crystalwas cooled to room temperature at a temperature decreasing rate of 20°C./hour, and the crystal ingot was taken out for machining.

Example 8

Preparation of 40 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.99% wereused as the starting materials. These starting materials were fullydried in a vacuum oven, and weighed in a molar ratio ofBaF₂:YF₃=0.60:0.40. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a glassy carbon crucible, andthen thoroughly melted in a vacuum furnace at 1350° C. to obtain aBaF₂—YF₃ solid solution melt. The melt was cooled to room temperature toobtain Ba_(0.6)Y_(0.4)F_(2.4) polycrystalline material.

3) A high-purity graphite crucible having a capillary structure at thebottom was processed according to the size and number of crystals to begrown, and the Ba_(0.6)Y_(0.4)F_(2.4) polycrystalline material and anappropriate amount of PbF₂ powder were fed into the graphite crucible,and placed into a vacuum crucible descending furnace.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to thoroughly melt the raw material, and adescending device was turned on for crystal growth, wherein thedescending speed was 0.8 mm/h. After the growth was completed, thecrystal was cooled to room temperature at a temperature decreasing rateof 15° C./hour, and the crystal ingot was taken out for machining.

Example 9

Preparation of 50 at % Y-Doped BaF₂ Crystal

1) BaF₂ having a purity of 99.99% and YF₃ having a purity of 99.99% wereused as the starting materials. These starting materials were fullydried in a vacuum oven, and weighed in a molar ratio ofBaF₂:YF₃=0.50:0.50. An appropriate amount of PbF₂ was weighed as adeoxidizer. BaF₂, YF₃, and PbF₂ were thoroughly mixed to obtain aBaF₂—YF₃—PbF₂ mixture.

2) The BaF₂—YF₃—PbF₂ mixture was fed into a glassy carbon crucible, andthen thoroughly melted in a vacuum furnace at 1360° C. to obtain aBaF₂—YF₃ solid solution melt. The melt was cooled to room temperature toobtain Ba_(0.5)Y_(0.5)F_(2.5) polycrystalline material.

3) A high-purity graphite crucible having a capillary structure at thebottom was processed according to the size and number of crystals to begrown, and the Ba_(0.5)Y_(0.5)F_(2.5) polycrystalline material and anappropriate amount of PbF₂ powder were fed into the graphite crucible,and placed into a vacuum crucible descending furnace.

4) A vacuum pumping device was turned on so that the vacuum degreeinside the furnace was less than 10⁻³ Pa, and then the temperature wasgradually increased to thoroughly melt the raw material, and adescending device was turned on for crystal growth, wherein thedescending speed was 0.5 mm/h. After the growth was completed, thecrystal was cooled to room temperature at a temperature decreasing rateof 10° C./hour, and the crystal ingot was taken out for machining.

In order to fully understand the invention, some specific technicaldetails and processes are described in the above examples, but theinvention may also be implemented in other ways than the abovedescription, and those skilled in the art can make similar expansionwithout departing the content of this invention.

The invention claimed is:
 1. A method for preparing an yttrium-dopedbarium fluoride scintillation crystal having a chemical composition ofBa_((1−x))Y_(x)F_(2+x), wherein 0.01≤x≤0.50, and the method comprisesthe steps of: weighing and mixing raw materials of YF₃ and BaF₂according to the molar ratio BaF₂:YF₃=(1−x): x to obtain a mixed powder,wherein 0.01≤x≤0.50; putting the mixed raw materials into crucibles in avacuum furnace for thorough melting at a temperature of 1200 to 1400°C., and then cooling the mixture to obtain Ba_((1−x))Y_(x)F_(2+x)polycrystalline material, or subjecting the mixed powder to isostaticpressing, and putting the resulting substance into crucibles andsintering it at 900 to 1200° C. in vacuum to obtain sinteredBa_((1−x))Y_(x)F_(2+x) polycrystalline material; and mixing theresulting polycrystalline material with an appropriate amount of PbF₂powder, which is act as a deoxidizer, and growing crystals by verticalBridgman method; wherein the processes of the vertical Bridgman methodinclude: maintaining the furnace in a vacuum degree of less than 10⁻³Pa, melting the Ba_((1−x))Y_(x)F_(2+x) polycrystalline material and PbF₂powder at 1200 to 1400° C., subjecting the resulting melt to crystalgrowth wherein the descending speed of the crucible is 0.5 to 4 mm/hour,and cooling the grown crystal to room temperature at a temperaturedecreasing rate of 10 to 50° C./hour; wherein the deoxidizer PbF₂ isadded in an amount of 0.5 to 5 wt % of the Ba_((1−x))Y_(x)F_(2+x)polycrystalline material.
 2. The method of claim 1, wherein thecrucibles are high purity graphite crucibles or glassy carbon crucibles.3. The method of claim 1, wherein the deoxidizer PbF₂ is added in anamount of 0.5 to 2 wt % of the Ba_((1−x))Y_(x)F_(2+x) polycrystallinematerial.
 4. The method of claim 1, wherein the isostatic pressing isperformed at a pressure of 5 to 20 MPa for 0.1 to 1 hour.